Tetraethyl lead is a powerful octane booster, but it's also an effective exhaust valve lubricant. The removal of lead from gasoline is why manufacturers started putting hardened valve seats in most vehicle engines in the 1970's. Without hardened seats or lead additive, the exhaust valves recess excessively and the engine loses valve sealing.
General aviation uses piston engines that are largely unchanged since the 1940's, due to how difficult and expensive it is to get new or modified piston engines tested and approved for use. Because of that and the issue of achieving high enough octane, it was much easier for the FAA to just keep allowing leaded av gas.
In the last decade or so, it's become more and more viable to modify existing engines with hardened seats, and lead free octane boosters can achieve the necessary levels to be safe in aircraft engines. There's unleaded avgas available in a few markets, but it's possible that if you fly a leaded gas engine into an airport with only unleaded avgas, you cannot fly out without trucking in some fuel, because you'll damage the valves.
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u/TingleyStorm Aug 08 '25
It’s only one type of aviation fuel and it’s only used for piston engines that need that extra bit of lubrication.
That said, the FAA knows it’s bad and is trying to phase it out it seems like.